Referring now to the drawings, wherein like features are designated by like reference numerals, FIG. 1 schematically illustrates one common type of prior-art, edge-emitting, separate-confinement-heterostructure, diode-laser 20 formed by epitaxially growing a series of semiconductor layers on an n-doped, semiconductor substrate 22. A lower electrical confinement or cladding layer 24 of the diode-laser is an n-doped semiconductor layer. Surmounting the lower cladding layer is a lower optical confinement or waveguide layer 26, an undoped semiconductor layer. Surmounting the lower waveguide layer is a quantum-well 28, also an undoped semiconductor layer. The active layer is surmounted by an upper waveguide layer 30, similar to lower waveguide layer 26. Upper waveguide layer 30 is surmounted in turn by an upper cladding layer 32, a p-doped semiconductor layer.
An elongated, rectangular electrode 34 is bonded to upper cladding layer 32. Electrode 34 extends the entire length (L) of the diode-laser. The width W and length L of the electrode define the width and length of the diode-laser. The electrode and the region under the electrode are often referred to by practitioners of the art to as a “stripe”.
The diode-laser is energized (electrically pumped) by passing current through the layers between electrode 34 and substrate 22. Mirrors 36 and 38 on the ends (facets) of the laser form a laser resonant cavity. Energizing the laser generates electrical carriers that recombine in the quantum-well to provide laser radiation that circulates in the resonant cavity. Laser radiation is emitted from the diode-laser in a general direction along a longitudinal (Z) axis of the laser. The radiation is emitted as a diverging beam (not shown) having a greater divergence in the Y-axis than in the X-axis. For this reason, the Y-axis and X-axis are respectively referred to as the fast and slow axes by practitioners of the art.
Laser radiation circulating in the laser cavity is confined in the thickness (Y-axis) direction of the layers by reflection from interfaces between the waveguide regions and the cladding regions adjacent thereto. The circulating radiation is confined in the width (X-axis) direction, among other factors, by the width of the electrode, as it is only in this width that there is optical gain.
This type of diode-laser typically has a stripe-length between about 1.0 and 1.5 millimeters (mm), and emits radiation from an emitting aperture (corresponding to the ends of layers 26, 28, and 30 covered by electrode 34) having a height H of about 1.0 micrometer (μm) and a width W between about 4.0 and 200 μm. The emitting aperture height H includes the combined thickness of the upper and lower waveguide layers 30 and 26 and the quantum well layer 28. Width W is usually referred to in the art as the emitter-width or stripe-width. A diode-laser having an emitter-width greater than about 30 μm is often referred to as a wide-emitter diode-laser.
Generally, for a given length of a diode-laser, the greater the stripe (emitter) width, the greater will be the potential output power of the diode-laser. However, the wider the stripe width, the greater is the number of transverse modes at which the laser delivers output radiation. The greater the number of transverse modes, the poorer is the quality of the output beam of the diode-laser. While a multiple transverse mode output beam is acceptable for diode-laser applications such a heating and surface treatment, it is often not suitable for applications in which the output beam must be focused into a small spot, for example in end-pumping a fiber laser. In a co-pending U.S. patent application Ser. No. 10/643,621, filed Aug. 19, 2003, assigned to the assignee of the present invention, a cause of the multiple transverse mode output of a wide stripe separate confinement heterostructure laser is identified as an uneven distribution of temperature across the width, i.e., in the X-axis direction, of the diode-laser. This uneven distribution or thermal gradient results from the passage of current through the laser stripe region. This thermal gradient is such that it causes a thermally induced phase curvature in the waveguide regions.
A computed, exemplary, such temperature distribution is schematically depicted, graphically, in FIG. 2. In this calculated example, it is assumed that the diode-laser is a semiconductor structure indium-soldered to a cooper heat-sink, the electrode has a width of 100 μm, and that 4 watts of current are passed through the diode-laser. The temperature peaks in the center (X=0) of the diode-laser stripe, i.e., on the Z-axis, and reduces towards the edges. This temperature distribution causes a corresponding refractive index variation across the width of the diode-laser. The refractive index distribution causes a phase curvature in the X-axis for radiation circulating in the waveguide regions of the laser.
A computed example of this phase curvature as a function of X-axis position is schematically, graphically depicted in FIG. 3. This phase curvature results in a 100-μm length of the laser having a dioptric power of about 0.033 mm−1, equivalent to a converging lens with a focal length of about 30.0 mm. Accordingly, the 1.0 mm length of the diode-laser provides the equivalent of a converging lens with a focal length of about 3.0 mm. A result of this thermally induced lens is that the fundamental operating mode of the laser is constrained to a relatively narrow central portion of the total width of the laser (the stripe width). This leaves gain in the remaining portions of the laser width available to support higher order operating modes, i.e., to support multiple transverse mode operation.
There is a need for a method of energizing a wide stripe, edge-emitting, separate confinement heterostructure semiconductor laser that does not provide the above discussed positive lens effect and allows the laser to operate in a single transverse mode.